In World War 2, on D-Day, the Canadian forces landed on a beach code-named "Juno". Where did the term "Juno" come from, in this instance?

The code names for the various D-Day landing beaches did not really "come from" anywhere in particular. All major military operations have code names so that they can be referred to in communications without the enemy (hopefully) knowing what they refer to. There are lists of code names and these names are typically given to operations randomly. This means that there was no particular reason to name that beach "Juno" and there was no particular reason for having the Canadians attack "Juno" rather than "Gold" or "Omaha."

So, all of the code names simply came from a list. They were not meant to have any particular meaning.

Juno was a Roman Goddess, who was mother of Vulcan and Mars. Her equivalent in Greek mythology was Hera. Vulcan was the god of beneficial and hindering fire, and Mars was the god of war.

Some of the etymology of 'Juno' includes the word iuvare- meaning to aid or benefit.

Juno is often depicted as a protectress, associated with fertility, government and military. Many associate her being a fertility goddess with the growth in numbers of a community relating to military actions.

The month of June was named after Juno, and that is when the landing took place. It was believed, with Juno as a goddess of marriage, that June is the luckiest month to be married in. It is possible that they also wished for luck in their mission.